Eclipse, in astronomy, complete or partial obscuring of a celestial body by another. An eclipse occurs when three celestial objects become aligned.

Total eclipse of the Sun occurring shortly after sunrise, in a composite photograph that shows …

Larry Landolfi/Photo Researchers, Inc.

An overview of eclipses of the Sun and the Moon.

Encyclopædia Britannica, Inc.

From the perspective of a person on Earth, the Sun is eclipsed when the Moon comes between it and Earth, and the Moon is eclipsed when it moves into the shadow of Earth cast by the Sun. Eclipses of natural satellites (moons) or of spacecraft orbiting or flying past a planet occur as the bodies move into the planet’s shadow. The two component stars of an eclipsing binary star move around each other in such a way that their orbital plane passes through or very near Earth, and each star periodically eclipses the other as seen from Earth.

When the apparent size of the eclipsed body is much smaller than that of the eclipsing body, the phenomenon is known as an occultation. Examples are the disappearance of a star, nebula, or planet behind the Moon or the vanishing of a natural satellite or spacecraft behind some body of the solar system.

A transit occurs when, as viewed from Earth or another point in space, a relatively small body passes across the disk of a larger body, usually the Sun or a planet, eclipsing only a very small area. Mercury and Venus, for example, periodically transit the Sun, and a natural satellite may transit its planet. Extrasolar planets (e.g., HD 209458b) have been discovered when they perform a transit of their stars.

Phenomena observed during eclipses

Lunar eclipse phenomena

The Moon, when full, may enter the shadow of Earth. The motion of the Moon around Earth is from west to east (see the figure of a lunar eclipse, in which the view of Earth is from above its North Pole). For an observer facing south, the shadowing of the Moon begins at its left edge (if the Moon were north of the observer, as, for example, in parts of the Southern Hemisphere, the opposite would be true). If the eclipse is a total one and circumstances are favourable, the Moon will pass through the umbra, the darkest part of the shadow, in about two hours. During this time the Moon is usually not completely dark. A part of the sunlight, especially the redder light, penetrates Earth’s atmosphere, is refracted into the shadow cone, and reaches the Moon. Meteorological conditions on Earth strongly affect the amount and colour of light that can penetrate the atmosphere. Generally, the totally eclipsed Moon is clearly visible and has a reddish brown, coppery colour, but the brightness varies strongly from one eclipse to another.

Before the Moon enters the umbra and after it leaves the umbra, it must pass through the penumbra, or partial shadow. When the border between umbra and penumbra is visible on the Moon, the border is seen to be part of a circle, the projection of the circumference of Earth. This is a direct proof of the spherical shape of Earth, a discovery made by the ancient Greeks. Because of Earth’s atmosphere, the edge of the umbra is rather diffuse, and the times of contact between the Moon and the umbra cannot be observed accurately.

During the eclipse the surface of the Moon cools at a rate dependent on the constitution of the lunar soil, which is not everywhere the same. Many spots on the Moon sometimes remain brighter than their surroundings during totality—particularly in their output of infrared radiation—possibly because their heat conductivity is less, but the cause is not fully understood.

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An eclipse happens when one object in space blocks another from view. For example, during a solar eclipse the Moon comes between Earth and the sun. The Moon blocks the sun for a time so that people on Earth cannot see it.

When three celestial objects become aligned, an eclipse is said to occur. The many eclipse events known to astronomers are of two different types. In the first, the eclipsing body comes between an observer and the eclipsed object. The eclipsed object appears to the observer to be totally or partially covered by the eclipsing object. Eclipses of the second type affect only planets or natural satellites. In this case, the eclipsing body comes between the Sun and the eclipsed object. The eclipsed object remains in view of the observer, but the Sun’s light no longer shines on any of it or part of it, and it becomes darkened by entering into the shadow of the eclipsing object. Examples of this kind of eclipse event are eclipses of the Moon and eclipses of the satellites of Jupiter.